Lithium-ion secondary batteries have been used as batteries of portable electronic devices for years. Such a lithium-ion secondary battery typically includes a positive electrode, a negative electrode, and a separator all disposed in an electrolytic solution. The positive electrode is formed by applying lithium cobalt oxide or lithium manganese oxide to the surface of an aluminum foil. The negative electrode is formed by applying carbon to the surface of a copper foil. The separator is disposed so as to separate the positive electrode and the negative electrode and prevents an electrical short circuit between the electrodes.
During charging of the lithium-ion secondary battery, lithium ions are released from the positive electrode and move into the negative electrode. During discharging of the lithium-ion secondary battery, lithium ions are released from the negative electrode and move into the positive electrode. Therefore, the separator needs to allow ions such as lithium ions to permeate therethrough.
As the separator, a synthetic resin microporous film is used because it has insulation properties and is inexpensive. The synthetic resin microporous film contains a synthetic resin, such as a propylene-based resin.
The synthetic resin microporous film is produced by stretching a synthetic resin film. The high residual stress due to stretching is generated in the synthetic resin microporous film produced by a stretching method. Thus, it has been pointed out that such a synthetic resin microporous film may thermally shrink under high temperature, which may cause a short circuit between the positive electrode and the negative electrode. Therefore, there is a need to ensure the safety of the lithium-ion secondary battery by improving the heat resistance of the synthetic resin microporous film.
Patent Literature 1 discloses, as a lithium-ion secondary battery separator, a synthetic resin microporous film that has been treated with electron beam irradiation and whose value obtained by thermomechanical analysis (TMA) at 100° C. is 0% to −1%.